Any time I see someone do that, I hear Homer Simpson in my mind. I remember episodes where he would just bust out in laughter, and it sounded just like that.
Bart and Homer are "innocent" bad boys...who seem like they might be soothed with a hug. Some of the other shows and movies that were inspired by the Simpsons took a darker turn.
When someone bwaahaas on FR, I general interpret it as kind of a verbal "mooning" (and am not much charmed).